1. Field of the Invention
The present invention relates to mounting systems for anti-lock brake sensors.
2. Description of the Prior Art
Electromagnetic sensors are commonly employed in vehicle anti-lock braking systems (ABS) as the basic element used for skid detection. The detection of skids is founded on detecting abrupt changes, particularly an abrupt decline, in the rotational velocity of a wheel. An electromagnetic sensor is used to generate a periodic or oscillating signal indicating the rotational rate of the wheel. Changes in the frequency of the oscillating signal, or the cessation of oscillation, indicate changes in wheel rotational speed.
The sensor cooperates with a disk mounted for rotation with a wheel being monitored. The disk is positioned so that one of its major surfaces faces the sensor, preferably with the sensor being adjacent the outside edge of the plate. Along the exterior edge on the plate face adjacent the sensor are alternating ridges and grooves. A magnet is positioned near the sensor on the opposite side of the disk. As a ridge moves between the magnet and sensor leakage flux is reduced with a consequent effect on the amplitude of the electrical signal produced by the sensor. As gaps between ridges become positioned between the magnet and the sensor the leakage flux increases with a corresponding inverse effect on the amplitude of the sensor output signal. A spinning wheel results in the rapid alternation of gaps and ridges moving between sensor and magnet and result in the sensor generating a signal of oscillating amplitude. When the wheel stops moving, as occurs when it locks up in a skid, the amplitude of the signal stops changing and occurrence of a skid is indicated.
Efficient operation of the sensor requires that the gap between the sensor head and the ridges on the rotating disk be minimized, while still allowing free, non-contacting movement of the wheel and plate. Excessive spacing between ridge and sensor head allows leakage of magnetic flux and, as a result, a signal with a substantially flattened amplitude changes is produced by the sensor in response to rotation of the plate. False indications of wheel lock-up then become possible.
A positioning mount for a skid sensor should keep the sensor in close proximity to the wheel once the sensor has been properly positioned. The mount should also assure ease in construction of the skid detection components with the wheel. Ease in construction is served by making the final assembly tolerant of minor mechanical variations and by making the sensor positionally self adjusting. Contemporary design has achieved the end of making the sensor positionally self adjusting by housing the sensor in an elongated cylindrical tube and then positioning the tube through a circular aperture through a bracket. The sensor housing is retained in the aperture by friction between a spring bushing which lines the interior circular aperture and which wraps around the sensor housing. The bushing should have a sufficient coefficient of friction with the sensor housing to require a minimum force of 25 lbs to move the sensor housing back and forth in the bushing. In assembly a technician can insert the sensor fully into the bracket until a stop on the sensor impinges on the body of the bracket. Then the wheel and plate assembly may be placed on the axle. The disk will come into contact with the sensor housing as the wheel is urged into position, and will force the sensor back to the appropriate position.
When the bushing is properly positioned in the bracket aperture, the bushing generates the required minimum force on the sensor housing automatically. However, in the prior art design problems have arisen with retaining the bushing in the aperture. If the bushing is improperly seated less force may be required to move the sensor.
The invention provides an improved bushing for insertion into a cylindrical aperture defined by a mounting bracket. The improved bushing provides for retention of a cylindrical sensor housing which is enveloped around its circumference by the bushing and positioned adjacent a rotating flux focusing element. The bushing is formed from a sheet of spring metal and shaped into a cylinder. A pair of outwardly projecting retaining brackets extend from a first end of the sheet and are positioned at one end of the cylinder. In addition, a pair of outwardly projecting dimples are formed in the sheet near the edge of the sheet opposite the first edge, the dimples being sized to allow insertion of the bushing into a cylindrical aperture, but to resist extraction of the bushing after its complete insertion through the cylindrical aperture.
Additional effects, features and advantages will be apparent in the written description that follows.